The invention relates to solar cell design and more particularly to multi junction solar cells employing dilute nitrides.
The highest solar cell efficiencies are known to be produced by multi junction (MJ) solar cells comprising III-V semiconductor alloys. Their relatively higher efficiencies make these devices attractive for both terrestrial concentrating photovoltaic systems and for celestial systems designed to operate in outer space. Multi junction solar cells have reached efficiencies up to 41.6% under concentrations equivalent to several hundred suns. Currently, the highest efficiency devices have three junctions and are either lattice matched to their substrate or contain metamorphic layers that are not lattice matched. Other factors equal, lattice-matched systems are preferable because they have proven reliability and require less semiconductor material than metamorphic solar cells, which require thick buffer layers to accommodate the differing lattice constants of adjacent materials.
Dilute nitrides are a class of III-V alloy materials (alloys having one or more elements from Group III on the periodic table along with one or more elements from Group V on the periodic table) with small fractions (e.g., <5 atomic percent) of nitrogen. These alloys are of interest for multi junction solar cells because they can be lattice matched to substrates of interest, including GaAs and Ge. Additionally, one can achieve a bandgap of 1 eV for the dilute nitride material, which is ideal for integration into a multi junction solar cell with substantial efficiency improvements.
GaInNAs, GaNAsSb and GaInNAsSb are some of the dilute nitride materials that have been studied as potentially useful for multi-junction solar cells (see, e.g., A. J. Ptak et al., Journal of Applied Physics 98 (2005) 094501 and Yoon et al., Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE, pp 76-80, 7-12, June 2009; doi: 10.1109/PVSC.2009.5411736). Furthermore, the use of four junction GaInP/GaAs/dilute-nitride/Ge solar cell structure holds the promise of efficiencies exceeding those of the standard metamorphic and lattice matched three-junction cell, which at present are the benchmark for high-efficiency multi-junction cell performance. (Friedman et al. Progress in Photovoltaics: Research and Applications 10 (2002), 331). To make that promise a reality, what is needed is a material that is lattice matched to GaAs and Ge with a band gap of near 1 eV and that produces open circuit voltage greater than 0.3 V with sufficient current to match the (Al)InGaP and (In)GaAs sub-cells in a multi-junction solar cell. It should be noted that a multi-junction solar cell for terrestrial use is integrated into a concentrated photovoltaic system. Such a system employs concentrating optics consisting of dish reflectors or Fresnel lenses that concentrate sunlight onto the solar cell. It is possible that a concentrator's optics may attenuate light in a particular wavelength region which may be detrimental to the dilute nitride sub-cell. It is therefore of utmost importance that higher current be generated in the dilute nitride sub-cell so any loss due to the concentrator optics does not inhibit the performance of the multi-junction solar cell.
In a multi-junction solar cell, each of the sub-cells is attached in series to other sub-cells, typically using tunnel junction diodes to connect the individual sub-cells to one another. Since the total current generated by the full stack of sub-cells must pass through all the sub-cells, the sub-cell passing the least amount of current will be the current-limiting cell for the entire stack, and by the same virtue, the efficiency-limiting cell. It is therefore of greatest importance that each sub-cell be current matched to the other sub-cells in the stack for best efficiency. This is particularly important if dilute nitride sub-cells are to be used because dilute nitride semiconductor materials historically have been plagued with poor minority carrier transport properties that prove detrimental when incorporated into a larger solar cell.
Although dilute nitride alloys have other properties that make them desirable for use in multi-junction structures, particularly the flexibility with which their bandgaps and lattice constants can be fine-tuned as part of their design, the minority carrier lifetime and diffusion lengths for these sub-cells are typically worse than with conventional solar cell semiconductors such as GaAs and InGaP used in conventional multi-junction solar cells, thus resulting in a loss of short circuit current, open circuit voltage or both. Moreover, the interface between the back surface field and the base of the dilute nitride sub-cell may have high surface recombination velocity, which could further reduce the short circuit current and open circuit voltage of the sub-cell. As a result of these problems, photocurrents generated in dilute nitride sub-cells are typically lower than with more traditional materials. (D. B. Jackrel et al., Journal of Applied Physics 101 (114916) 2007).
Dopant variation in solar cells is known. See M. A. Green, Progress in Photovoltaics: Research and Applications 17 (2009). U.S. Pat. No. 7,727,795 is an example of a solar cell design using exponential doping in parts of a solar cell structure, evidently for multi-junction solar cells grown in an inverted metamorphic and lattice mismatched structure. The application to dilute nitride sub-cells is not suggested and is not obvious, due to the anomalous characteristics of dilute nitrides. Dilute nitrides are a novel class of materials, which frequently exhibit different behavior than seen in traditional semiconductor alloys. For example, bandgap bowing as a function of alloy composition is very different in dilute nitrides as compared to traditional semiconductors (e.g., Wu et al., Semicondutor Science and Technology 17, 860 (2002)). Likewise, the standard dopants and doping profiles used for traditional semiconductors such as GaAs and InGaP do not result in comparable characteristics in dilute nitride semiconductors. For example, dopant incorporation in dilute nitrides has anomalous behavior. A Yu et. al. paper reported that when dilute nitride thin films are doped heavily with Si, the Si and N mutually passivate each other's electronic activity (Yu et. al. App. Phys. Lett. 83, 2844 (2003)). Similarly, Janotti et. al. (Phys. Rev. Lett. 100, 045505 (2008)) suggested that while the physics of n-type and p-type doping in the parent compounds GaAs and GaN is well established, doping in GaAs1-xNx is much less explored and the interaction between extrinsic dopants and N in GaAs1-xNx alloys can lead to entirely new phenomena. They also pointed that rapid thermal annealing of Si-doped dilute (In)GaAsN alloys at temperatures above 800° C. leads to a drastic increase in the electrical resistivity. Due to the uncertainties associated with doping profiles and outcomes, and due to the unique properties of dilute nitrides, it is not apparent to one of ordinary skill how the concepts taught therein could be incorporated into a solar cell employing dilute nitride elements having portions subjected to controlled doping. Moreover, due to difficulties in doping the dilute nitride alloys, the literature teaches that dilute nitride alloys should not be doped (i.e., should be intrinsic) when incorporated into solar cell structures, for enhancement of the current collection (e.g., Ptak et al. J. Appl. Phys. 98, 094501 (2005); Volz et al. J. Crys. Growth 310, 2222 (2008)). Rather, the literature teaches that the use of doping in the base of the dilute nitride solar cell leads to decreased performance.
Known as well, as previously discussed, dilute nitride cells were thought to have significant drawbacks such that their incorporation into multi-junction solar cells would have led to substantial loss in the efficiency of such a solar cell, thus making dilute nitride cells less attractive commercially than other types of materials. It is desirable to improve current collection in dilute nitride based sub-cells without an accompanying loss of short circuit current, open circuit voltage or both.